A single-phase motor uses a start capacitor to start the motor. The start capacitor and the auxiliary winding are capable of only intermittent use. The single-phase motors use a switch to disconnect the current to the capacitor once approximately 75% of the motor's synchronous speed is reached. In the art, a mechanical switch achieves this.
Mechanical switches operate on the centrifugal principle. The switch opens up at a predetermined speed, which is required to initiate centrifugal action, and then detaches the capacitor from the motor circuit thereby closing the power supply thereto. When switch falls below the predetermined speed, the switch closes (centrifuge action) attaching the capacitor to the circuit. The mechanical switch therefore attaches and detaches the capacitor for capacitor start motor.
Single-phase motors typically require the inclusion of start winding to cause the main winding in the motor to produce magnetic fields having a rotational character. Once started, the rotating rotor will itself give rise to induced magnetic fields such that a two-pole run winding is capable maintaining the rotation of the rotor. The start winding is energized for only a short period of time following start-up to permit the motor to be brought up to operating speed and which is thereafter de-energized.
Prior known electronic switch measure voltage or current flow through the start winding to determine switching from the start winding to the run winding.
The utilization of solid state switches for motor starting to improve reliability and longevity over conventional electromechanical relay is well known.
A solid-state switch provides an alternate solution for capacitor connection and disconnection, to the mechanical switch with vital added advantages.
The basic principle of operation for a solid state switch for a single phase induction motor having a main and auxiliary winding is based on the principle that a 90° out of phase voltage is induced in the auxiliary winding whose amplitude is proportional to the speed of the motor and is almost equal to the mains voltage at synchronous speed, that means phase angle between mains voltage (M) and vector sum of auxiliary and mains voltage will almost equal be equal to zero at zero speed and almost equal to 45° at a synchronous speed.
Supply to the auxiliary winding is provided through the ‘solid state switch’. Solid-state switch disconnects the auxiliary winding supply when this angle reaches 36°. Also the solid-state switch reconnects the auxiliary winding supply if this phase vector angle is less than 18°. This solid-state switch to perform switching action of capacitor current senses the phase angle continuously.
Unlike mechanical switch, solid-state switch varies only with frequency of AC supply, thus the same solid-state switch can be used for different speed motors. However, in this case of mechanical switches, different speed motors require different mechanical switches. Another advantage of solid state switches is that they can be mounted anywhere including outside the motor or even distant from the motor. Mechanical switches on the other hand have to be mounted on the motor shaft, thereby increasing the size of the motor.
The conventional solid state switch sense current by a current sensor through the main winding and then actuate the disconnect switch at a designated condition. This approach may be objectionable due to the extra wattage and heat because current is still flowing through the sensor in the run mode after starting.
To overcome the above mentioned limitation U.S. Pat. No. 4,622,506 provides a system for an electronic control system for an auxiliary winding disconnected switch which eliminate the need for extra component on the motor shaft or in series in the motor circuit. However, this system is load sensitive.
U.S. Pat. No. 3,414,789 and U.S. Pat. No. 3,671,830 switching system utilize one or more characteristic of the motor to prevent conduction of the triac and hence effect deenergization of the start winding at the optimum moment. However, variation in voltage supply and loading effect the motor speed at which the start winding is deenergized resulting in inconsistent performance.
U.S. Pat. No. 4,496,895 provides a motor starter which can be used with both split-phase and capacitor start motors and which can be used with a whole class of motors without any need for tailoring to a specific application. This motor starter has a positive lockout capability to prevent reenergization of the start winding until the motor is deenergized. However, the pulse transformer used in this system is very expensive which makes the switch costly.
U.S. Pat. No. 4,670,697 provides a low cost switch for use with the start or auxiliary winding of a single phase AC induction motor. However, this switch is load sensitive.
U.S. Pat. No. 6,756,756 provides a simple, low cost motor starting controller that is speed sensitive and AC line voltage insensitive. However, the switch is load sensitive and it does not check the sped of the motor before reenergization.